As working machines for cutting grass or plants, bush cutters in which a rotary blade is driven are known. Although bush cutters using an internal combustion engine as a power source has been generally used, recently, electric bush cutters are practical and widely used due to an improved performance of rechargeable secondary cells. If an electric power is used as the power source of the bush cutters, there are features in that noise and exhaust gas are not emitted and running costs are excellent. Such electric bush cutters have been disclosed in Patent Document 1, in which a rotational number of a motor can be adjusted. A configuration of such an electric bush cutter according to the related art will be described with reference to FIG. 17. In an electric bush cutter 1001 driven by a battery pack 1002, a motor unit 1004 is attached to an end portion of a pipe shaped rod 1006 and a cutting blade 1005 is rotated by driving a motor (not shown) accommodated in the motor unit 1004 by the battery pack 1002. A scattering protection cover 1011 for preventing cut grass from being scattered is provided in the vicinity of the cutting blade 1005. The electric bush cutter 1001 is carried by a shoulder sling belt or the like not shown and includes a handle pipe 1007 for operation by a worker having an approximately U-shape when viewed form the front and attached to the vicinity of the center of the rod 1006 in a longitudinal direction of the rod 1006. Also, grip portions 1008a and 1008b are provided on both ends of the handle pipe 1007. A waist abutting member 1012 abutted to a waist of a worker during a work is provided in the vicinity of a portion of the rod 1006 between the handle pipe 1007 and an operation unit 1003. Rotation of the motor unit 1004 is controlled by a worker using a trigger lever 1009 attached on the grip portion 1008a. A rotational number of the motor unit 1004 is changed by varying a voltage applied to the motor and is adjusted by a dial 1010. In this way, the rotation velocity of the blade can be changed depending on an object to be cut and the shape thereof.